1. Field of the Invention
This invention relates to the viscosity of certain polymers and, in particular, but not exclusively, to the use of such polymers in reconstituted tobacco products or tobacco substitutes.
2. Brief Description of Related Art
In tobacco reconstitution processes it is well known that suitable reconstituted products can be produced without added adhesives provided that the innate pectinaceous material in the tobacco stem is released. This pectin release is achieved by stem `cooking` at about 100.degree. C. or more for 1-2 hours followed by a further mechanical treatment to yield a material known informally as the stem binder.
However, the product thus produced can be further improved if a small amount, say about 1%, of cellulosic binder material, such as sodium carboxymethyl cellulose, is added. This provides easier processing as well as a stronger final product.
In the case of reconstitution of tobacco sheet by routes other than stem cooking, much higher amounts of non-tobacco binders, usually cellulosic derivatives, are required since no pectin release is involved. The level of binder usage in such products varies, depending on the cellulosic derivative chosen and the required end properties, but is generally within the range of about 5% to about 15%. A disadvantage of such products is the high proportion of binder required, especially if one is seeking to provide an all-tobacco, or substantially so, reconstituted material. Furthermore, the smoke taste characteristics of some binders are often less than desirable. This is particularly the case with sodium carboxymethyl cellulose, (SCMC) for example.
From work which has been undertaken with respect to reconstituted and synthetic products we have identified various alginates, which are cellulose binders derived normally from seaweed sources, which offer satisfactory processability and product strength, but which also have a much more acceptable smoke character than many of the other cellulosics.
Whilst working in the area of all-tobacco reconstituted products, it was found that the stem binder system or mix, which would enable the utilisation of a tobacco derived binder material and very low levels of non-tobacco cellulosics, was only really able to produce sheet material product by conventional flat plate or sheet/band casting methods. Attempts to cast a similar consistency mixture on a horizontal axis rotating drum caster, using a gate coater at the top of the drum, were not successful. Dilution of the slurry to a consistency, and thus viscosity, sufficient to flow under the gate coater when set at the desired height for final product thickness caused the slightly dried, hot slurry to run down the drum. Attempts to cast at a higher consistency also failed since the slurry would not then flow evenly under the gate coater.
Since the gross slurry viscosity for this binder mix was similar to that of other slurries successfully cast on the drum caster, this effect was somewhat surprising. The reason for this is thought to be due to the occurrence of extremely high local viscosity in the areas close to the tobacco and other solid particles, yet low viscosity in the aqueous solution between the particles.
It was then found that beneficial casting effects and a pronounced increase in product strength were obtained by the inclusion of relatively low levels, about 7%, of propylene glycol alginate (PGA) to the slurry. However, in view of the unexpected problems experienced in the drum casting process and our postulated explanation we wished to try and ensure that a truly homogeneous slurry was produced. Rather than use the relatively low shear mixing system as previously utilised, in a purely speculative trial it was decided to run a slurry batch through an APV Gaulin Lab 60 laboratory valve homogeniser at 2000 psi (13600 kPa) to give high shear conditions.
Very surprisingly, it was found that the product slurry exhibited a considerable increase in viscosity over the input material. This viscosity increase allowed very easy and successful casting of an otherwise difficult product. Another particularly beneficial advantage was then identified in that the level of non-tobacco binder required to achieve a desired product strength could be decreased. This benefit can be important in order to keep the level of non-tobacco additives to a minimum.
Further work has identified a number of polymers which exhibit this surprising feature. Physico-chemically, the phenomenon is unusual in that polymers in general react poorly to highshear forces, such as those applied by an homogeniser. Prior teaching and knowledge leads one to expect that the shear work done in the homogeniser is much more likely to break the polymer chains, thus lowering the average molecular weight giving a consequent expected viscosity loss. The identified polymers react contrary to this theory.